|Abstract||This National Standard Method (NSM) describes the identification of Shigella species with particular reference to isolation from faeces.|
The genus Shigella belongs to the family Enterobacteriaceae and consists of four species; Shigella dysenteriae, Shigella flexneri, Shigella boydii, and Shigella sonnei. Each of the species, with the exception of S. sonnei, is subdivided by serotype.
Shigella species are small Gram-negative rods. They produce pink colonies on XLD medium and
colourless colonies on DCA. Shigella species are facultative anaerobes, are non-motile, oxidasenegative, urease-negative, do not decarboxylate lysine and all except S. dysenteriae type 1 are catalase-positive. The species may be differentiated by biochemical tests and serology of their lipopolysaccharides. The majority of Shigella species, except S. flexneri 6, and
S. boydii 13 and 14, ferment sugars without gas production. S. boydii, S. flexneri and S. sonnei, with a few exceptions, ferment mannitol; S. dysenteriae does not. S. sonnei, and S. dysenteriae type 1 are the only species that are ONPG-positive. S. boydii 13 are Ornithine positive, and some may be ONPG positive.
Shigella species are highly infective. The infective dose is particularly low with S. dysenteriae, which may require as few as 10-100 organisms to cause infection.
Principles of identification:
Isolates from primary culture are identified by colonial appearance, biochemical tests and serology (agglutination with specific antisera). Plesiomonas shigelloides cross reacts with S. sonnei antisera. If confirmation of identification is required, isolates should be sent to the Reference Laboratory. All identification tests should ideally be performed from non-selective agar.